π» Urine Output Calculator
Is the kidney making enough urine?
Urine output rate
For informational purposes only, not medical advice.
Normal urine output is at least 0.5 mL per kilogram per hour. For a 70 kg adult making 300 mL over 8 hours, the rate is 300 divided by 70 divided by 8, or 0.54 mL/kg/hr, just inside normal. Below 0.5 is oliguria, and under about 100 mL per day is anuria: both are kidney warning lights, not variations of normal.
Urine output is the cheapest continuous kidney monitor in medicine, which is why catheters and hourly charts exist. Creatinine lags injury by hours; output drops in real time.
How to Use This Calculator
Enter the collected urine volume, the patient's weight, and the collection period. The tool normalizes to weight and time and classifies the result.
Enter the total urine volume in milliliters for the collection period.
Enter weight in kilograms and the hours the urine was collected over.
Read the mL/kg/hr rate and the classification. Oliguria persisting over hours needs clinical attention.
The Thresholds
The 0.5 threshold is one of medicine's most durable numbers, marking the point where the kidney can no longer clear the daily solute load in the water provided. Oliguria for more than 6 hours is itself a diagnostic criterion for acute kidney injury, independent of creatinine. Anuria, essentially no urine, points at complete obstruction, catastrophic perfusion failure, or end-stage kidneys.
Weight normalization matters: 30 mL/hr is fine for a 50 kg patient (0.6) but oliguric for a 100 kg one (0.3). Raw milliliters mislead; the per-kilo rate tells the truth.
Worked Example
A 70 kg patient makes 300 mL over an 8-hour night shift: 300/70/8 = 0.54 mL/kg/hr. Normal, though at the low end, worth watching but not alarming on its own. Had the same patient made 150 mL, the rate would be 0.27, clear oliguria demanding assessment of volume status, blood pressure, and medications.
Context decides urgency: oliguria in a dehydrated marathon runner resolves with fluids, while oliguria with rising creatinine and no response to a fluid challenge suggests intrinsic kidney injury. The number raises the question; the exam answers it.
Reading Your Result
| Rate | Classification |
|---|---|
| Under 0.1 mL/kg/hr | Anuria range. Emergency evaluation for obstruction or catastrophic causes. |
| 0.1 to 0.49 | Oliguria. Assess volume, perfusion, nephrotoxins, and obstruction. |
| 0.5 to 1.0 | Normal, low-normal at the bottom edge. |
| Over 2.0 | High output. Consider diuretics, diabetes insipidus, or osmotic diuresis. |
My take: trend beats snapshot. A patient drifting from 1.2 to 0.6 over a day is deteriorating even though every value stayed normal, while a stable 0.55 in a small elderly patient may be their baseline. Plot the rates, not just the latest.
Limits
Output can be normal while kidneys fail (non-oliguric AKI), so a good rate never rules out injury when creatinine is rising. Diuretics fake good output from sick kidneys. And collection errors, missed voids, spilled jugs, are the commonest cause of alarming numbers on the ward; verify the collection before escalating.
Related Tools
Fluid balance starts with intake: the daily water intake calculator sets the input side, the blood pressure checker tracks the perfusion driving output, and the BMI calculator frames body size.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
What is normal urine output?
At least 0.5 mL per kilogram per hour in adults, or roughly 800 to 2,000 mL per day. For a 70 kg adult that means 35 mL per hour minimum. Output is the real-time monitor of kidney perfusion.
What is oliguria?
Urine output below 0.5 mL/kg/hr. Oliguria lasting more than 6 hours is a criterion for acute kidney injury. Causes divide into prerenal (low perfusion), intrinsic (kidney injury), and postrenal (obstruction).
What is anuria?
Essentially no urine, under 100 mL per day. It suggests complete urinary obstruction, bilateral catastrophe, or end-stage kidney failure, and it is always an emergency.
How do you calculate urine output per kg per hour?
Divide total milliliters by weight in kilograms, then by hours collected. 300 mL over 8 hours in a 70 kg adult is 300/70/8 = 0.54 mL/kg/hr.
Can kidneys fail with normal urine output?
Yes, non-oliguric acute kidney injury is common, especially with nephrotoxins. Normal output does not rule out injury when creatinine is rising or the clinical picture suggests it.
Do diuretics affect urine output interpretation?
They raise output pharmacologically, which can mask underlying kidney dysfunction. A good rate on diuretics deserves less reassurance than the same rate without them.
What causes high urine output?
Diuretics, uncontrolled diabetes (glucose pulling water), diabetes insipidus, high fluid intake, and recovery phases of kidney injury. Persistent polyuria with thirst needs evaluation.
How is urine output measured in hospitals?
With an indwelling catheter and hourly recording on a fluid balance chart. Accurate measurement matters because management decisions, fluids, diuretics, dialysis timing, hinge on the trend.
Does low urine output always mean dehydration?
No. Dehydration is one prerenal cause, but obstruction, kidney injury, and low blood pressure from any cause also drop output. The exam and labs separate them; fluids alone are not always the answer.
When should low urine output worry me?
Oliguria persisting several hours, especially with dizziness, swelling, confusion, or no improvement after drinking fluids, warrants prompt medical evaluation. Anuria is an emergency.